European patent application No. 09154493.2, in the name of the same applicant as the present application, was filed before and published after the priority date of the present application. It describes a system arranged to process a sequence of images. The system carries out this processing in order to determine at least one of the presence and a frequency value of at least one peak in the spectrum of a signal based on the pixel data of the images corresponding to a frequency of a periodic physiological phenomenon. An image segmentation algorithm is performed on at least one of the sequence of images. One or more distinct segments determined to correspond to a body part of a desired type are tracked through the sequence of images. For each selected and tracked segment a measurement zone is selected. For each measurement zone, a signal representative of the time-varying average brightness of the pixels corresponding to the measurement zone is generated. The brightness may be a weighted sum of the color components or only the value of one color component. The signal is centered on its mean value, and basic signal processing techniques are used to determine one or more local maxima of the spectrum of the signal, at least within a range known to comprise typical heart rate values and/or respiration rate values for the living beings of interest (generally humans).